WALLA WALLA, Wash. - Not since Jennifer Lopez has a backside been studied as intently.

Hours after a team of researchers laid out apples scented with animal odor and blanketed a lush forest near Mount St. Helens with infrared beams - something took the bait.

In the mud beneath the high-tech trap, that something left an imprint of a gigantic rear end. To almost anyone walking along the Cascade Range in Washington that day several months ago, the imprint could have appeared to be an elk's or a bear's. But to the group of hunters on a quixotic expedition, it was the stamp of a legendary, elusive hominid.

Bigfoot, that is.

''The buttock impression is quite distinctive. You can see one of the cheeks quite clearly,'' said Jeff Meldrum, an Idaho State University anthropologist who flew in to study it, examining the depth and roundness of the find.

While the hunt for Bigfoot has all but died in most parts of North America, it has exploded into an enthusiastic, high-tech search throughout the Pacific Northwest. Highly organized and armed with infrared cameras, computers, videos, night scopes, and software, this new breed of hunters - led by Meldrum with his doctorates and other academic credentials - believe they will prove to the world that the primate exists.

''There is a creature out there we don't know very much about,'' said LeRoy Fish, a retired ecologist in Oregon who discovered the buttock imprint.

Meldrum, who discusses the ridge pattern of the large footprints he's collected the same way some people discuss their classical music collection, brings a touch of seriousness and order to a subject that for years was kept alive by breathless stories told by farmers and by unsubstantiated reports of wailings in the night heard by rural schoolboys. Now Meldrum and Fish say the Bigfoot mystery will only be explained through methodical research.

In Oregon, Fish, 58, is working with several molecular biologists to try to capture Bigfoot DNA from hair samples collected in the wild. If successful, he says, the scientists might be able to determine whether they're looking for a great ape or a hominid.

Despite all the hours Fish spends looking for Bigfoot's hair and Meldrum spends studying the casts of 200 footprints, which he says are mostly hoax-free, they can't escape the snickers.

''There are some people who feel I am on a wild goose chase,'' said Meldrum, 43. ''People can ignore it but they can't deny it. With these footprints one has to propose a hypothesis:

''They exist because of a hoax, because someone misidentified them, or because they belong to some unrecognized animal.''

It has been a lonesome journey for the college faculty member and a few other scientist-types who have joined the ranks of the 800-member Bigfoot Field Research Organization.

While Meldrum's reputation rises to star status among Bigfoot enthusiasts (one affluent resident in Washington State donated $50,000 to his research), he's losing credibility among fellow anthropologists. They question the time he spends investigating a phantom subject. Or why a distinquished scientist would climb trees in the middle of the winter looking for a mythical creature. One colleague said Meldrum is judged harshly during peer reviews and passed over when he tries to publish his Bigfoot findings in scholarly journals.

''One colleague said `after all, Jeff, these are just stories,''' Meldrum recalled. Bigfoot is ''certainly an icon, but it's much deeper than that here.''

True, the legend of Bigfoot runs as long and deep as the region's river canyons. While it has been 33 years since Roger Patterson toured the Northwest with his famous film of a hairy ape walking upright across a sandbar, the creature has become part of the area's culture. It has spawned Bigfoot hamburgers, pizza, tires, trucks, and museums.

Everyone from the maintenance man who works at Walla Walla's finest hotel to Brian Smith, a 32-year-old apartment manager, claims to have glimpsed a Bigfoot.''Two of them walked right past me,'' Smith said.

That happened, he said, 16 years ago. These days, grown men like Smith can be found traipsing with their high-tech equipment to Washington's mountains or disappearing into the forests of Oregon and Canada.

On one recent sunny day, Smith, who conducts searches at Meldrum's behest, drove 4,000 feet up into the Blue Mountains, where he had the sighting.

''A Bigfoot could be looking at us right now,'' Smith said as he unloaded his search gear from the bed of his truck.

He has traveled there so many times he knows exactly where he left a string of dried fruit, or on which trees he has hung a camera that clicks when a warm body passes by. Smith ripped open a batch of developed photos: All he found was a picture of the frightened face of a fawn.

''I wish he'd find him,'' said his wife, Tracy, who is tired of his leaving the house in the middle of the night to go searching for Bigfoot.

''As long as it's a mystery, there will be people interested in it,'' said Robin Ridington, a cultural anthropologist at the University of British Columbia who doubts Bigfoot is really out there - anywhere.

''Why should I believe in something in which the evidence is extremely shaky?'' Ridington said. ''If you recognize there's an actual species, then you run into problems. What would it feed on?''

But all Smith knows is what he saw and, for that reason, he will keep searching and reporting to Meldrum.

''In my mind, among the Bigfoot community, Meldrum is the top dog,'' said Smith.

Meldrum once believed Bigfoot was nothing more than a myth, too. He changed his mind one morning six years ago when he traveled to the Blue Mountains, the setting of a book he was reviewing. One of the people in the book took him to see what was supposed to be a fresh batch of Bigfoot prints.

''My first thought was how did this man do this, how could he possibly have created this hoax?'' Meldrum recalled.

As he stood that day staring at 40 tracks of 15-inch footprints in the mud, he envisioned what happened: Something scurried along the muddy road as if something spooked it. Then turned around, retreating back to the woods and leaving a trail of footprints.

''I said `something walked by here last night with a human- looking foot and toes,''' he recalled.

When he talks about that moment, he takes a deep breath.

''The closest feeling is when you hold an exquisite fossil in your hand,'' he said, dreamily. ''That same kind of feeling comes over you.''